1. Field of the Invention
The present invention relates to an object, a method, or a manufacturing method. The present invention relates to a process, a machine, manufacture, or a composition of matter. One embodiment of the present invention relates to a semiconductor device, a display device, a light-emitting device, a power storage device, a driving method thereof, or a manufacturing method thereof. One embodiment of the present invention relates to a positive electrode active material, a secondary battery, and a manufacturing method thereof. In particular, one embodiment of the present invention relates to an electrode for a power storage device and a manufacturing method thereof.
2. Description of the Related Art
In recent years, portable electronic devices such as mobile phones, smartphones, electronic book (e-book) readers, and portable game machines have been widely used. Being used as power sources for driving these devices, power storage devices typified by lithium-ion secondary batteries have been researched and developed actively. Power storage devices are of growing importance in a variety of uses; for example, hybrid vehicles and electric vehicles receive attention because of an increased interest in global environmental problems and an oil resources problem.
A lithium-ion secondary battery, which is a power storage device and widely used because of its high energy density, includes a positive electrode including an active material such as lithium cobalt oxide (LiCoO2) or lithium iron phosphate (LiFePO4), a negative electrode formed of a carbon material such as graphite capable of receiving and releasing lithium ions, and an electrolytic solution in which an electrolyte formed of a lithium salt such as LiBF4 or LiPF6 is dissolved in an organic solvent such as ethylene carbonate (EC) or diethyl carbonate (DEC), for example. The lithium-ion secondary battery is charged and discharged in such a manner that lithium ions in the lithium-ion secondary battery move between the positive electrode and the negative electrode through the electrolytic solution and intercalated into or deintercalated from the positive electrode active material and the negative electrode active material.
Lithium-ion secondary batteries are widely used as power sources for driving portable electronic devices, electric vehicles, and the like, and there is a very great need for more compact and higher capacity lithium-ion secondary batteries.
Thus, electrodes formed of an alloy-based material of silicon, tin, or the like, instead of a carbon material such as graphite (black lead) that has been conventionally used as a negative electrode active material, have been actively developed. A negative electrode used in a lithium-ion secondary battery is fabricated by forming an active material on one surface of a current collector. Graphite that can receive and release ions serving as carriers (hereinafter referred to as carrier ions) has been conventionally used as a negative electrode active material. The negative electrode has been fabricated in such a manner that graphite as a negative electrode active material, carbon black as a conductive additive, and a resin as a binder are mixed to form slurry, and the slurry is applied to a current collector and dried.
Compared with carbon, silicon, which is a material alloyed and dealloyed with lithium, can increase capacity when used as a negative electrode active material. The negative electrode of carbon (graphite) has a theoretical capacity of 372 mAh/g, whereas the negative electrode of silicon has a dramatically high theoretical capacity of 4200 mAh/g, and thus silicon is an optimum material for higher capacity lithium-ion secondary batteries.
However, when the material that is alloyed and dealloyed with lithium (e.g., silicon) greatly expands and contracts with reception and release of carrier ions in charge and discharge cycles; therefore, when the amount of carrier ions received by the material increases, the contact state between an active material and a conductive additive, between active materials, and between an active material and a current collector becomes worse and a conductive path is lost in some cases. The loss of the conductive path decreases the capacity with charge and discharge cycles. Moreover, in some cases, silicon is deformed or broken to be separated from a current collector or pulverized, so that a function as a lithium-ion secondary battery becomes difficult to maintain.
In Patent Document 1, a silicon layer is formed over a current collector, and a conductive layer is formed over the silicon layer. This allows electrical connection between the silicon layer and the current collector to be maintained through the conductive layer even when the silicon layer is separated from the current collector because of repeated expansion and contraction of the silicon; thus, degradation of battery properties can be inhibited. Patent Document 1 also discloses that a silicon layer to which an impurity such as phosphorus or boron is added is used as the conductive layer.